A hollow fiber contactor is mainly composed of a bundle of hollow fibers inside a contactor shell with both free ends potted in epoxy resin. The hollow fibers can be made of several materials such as polypropylene, cellulose acetate, and polyamide. Methods for manufacturing these contactors are described elsewhere by suppliers such as Du Pont, Dow, Monsanto, Hercules, and Celanese. Some of the recent patents describing ways of manufacturing these contactors include U.S. Pat. No. 5,470,469 by E.I. Du Pont de Nemours and Company, U.S. Pat. No. 6,616,841 by Celgard Inc., U.S. Pat. No. 5,695,545 by Hoechst Celanese Corporation, and U.S. Pat. No. 5,695,702 by Millipore Corporation.
Evaporator condensates from pulp mills contain several hazardous and undesirable compounds which often prevent the mill from recycling these streams to the fibreline and/or chemical recovery. These streams are unique and different in terms of composition and diversity from any other aqueous solutions treated using hollow fiber contactors. In fact, in kraft foul condensate more than 60 different compounds have been detected. To help pulp mills further advance towards system closure, condensates must be treated before being used to displace fresh hot water in various pulp mill operations. The major total reduced sulphur (TRS) compounds found in evaporator condensates from kraft pulp mills are: hydrogen sulphide (H2S), methyl mercaptan (CH3S), dimethyl sulphide (CH3SCH3) and dimethyl disulphide (CH3SSCH3). Other chemical compounds, present in evaporator condensates that contribute to organic loading to the effluent treatment system of pulp and paper mills (usually reported as Biochemical Oxygen Demand (BOD) or Chemical Oxygen Demand (COD)) include: methanol, ethanol, acetone, methyl ethyl ketone, terpenes, phenolics, and resin acids. Sulphite mill condensates contain a significant amount of SO2 as well as other undesirable organic compounds (e.g. furfural, acetone, acetic acid, diacetyl, furfural mercaptan and furfural methyl sulphide). Most of these undesirable compounds have an unpleasant or repulsive odour and exposure of mill workers to them could lead to several health-related problems. Therefore, the need exists to develop cost-effective approaches for removing these compounds from pulp mill condensates:
At present, most kraft pulp mills use air stripping to remove most of the TRS compounds or steam stripping to remove both TRS and methanol from evaporator condensates. Other kraft mills sewer their condensate which causes odour problems in the proximity of the mill and/or excessive hydraulic and organic loading to the effluent treatment system. Air stripping requires tall and expensive columns and may lead to operational problems such as foaming, flooding or channeling. Steam stripping is usually associated with high capital and operating costs since live steam is employed in most cases. In the case of steam stripping, the steam-to-condensate mass ratio needed for adequate TRS and methanol removal is between 15 to 20% on a mass basis. Other approaches such as the thermal oxidizer technology or chemical addition can be employed to destroy or mask the odour of the odorous compounds; however, these alternatives are associated with high capital and/or operating costs. On the average, the overall amount of condensate produced by a pulp mill can be about 8500 L/tonne of pulp produced. A typical North American mill produces about 1000 tonne of pulp per day. In most cases, only a fraction of the condensate is treated in a steam stripper for TRS and methanol removal.
U.S. Pat. No. 6,110,376 describes a method to treat evaporator clean condensate using a reverse osmosis (RO) membrane system. This technology is used in one kraft mill in North America. However, RO requires high pressures and large membrane areas to achieve the required high fluxes. U.S. Pat. No. 4,952,751 deals with the treatment of evaporator condensates using pervaporation. A silicone rubber membrane was used to remove methanol from black liquor evaporator condensate. No other odorous or TRS compounds were considered for removal from this stream. None of the aforementioned approaches included the use of hollow fiber membrane contactor technology for the treatment of pulp mill evaporator condensates.
Hollow fiber contactors can be used to treat pulp mill condensates efficiently and economically. These contactors contain several fibers into the interior of which an aqueous feed solution is allowed to flow while an acceptor fluid, or stripping agent, for example, another aqueous solution or a gaseous carrier, is allowed to flow on the outside of the fibers. These contactors can provide fast mass transfer without flooding or loading since the acceptor fluid and the aqueous feed solution flow on different sides of the hollow fiber and, hence, can be better controlled. Fast mass transfer in these hollow fiber modules is assured by the high surface membrane area per unit volume. It has been shown that the mass transfer of volatile compounds in hollow fiber contactors, in which air is used as the stripping medium, can be an order of magnitude greater than that achievable in packed towers. In summary, the use of these contactors has several advantages over air stripping:                Lower air flow rates when air is used as the stripping agent in the hollow fiber contactor.        The flow of the aqueous feed solution and the stripping agent (aqueous solution or gaseous carrier) can be controlled separately since a membrane separates the two.        No aqueous flow channeling.        